Global Pose/Rotation of a bone
If I understand you right, the bone shall have the same rotation as its parent (in pose mode):



To achieve this by script, all you basically need is the rotation part of the parent. Construct a 4x4 transformation matrix from the parent rotation and the target bone's translation and scale. Assign it to the bone's matrix property.

import bpy
from mathutils import Matrix

pbone = bpy.context.active_pose_bone
pbone_parent = pbone.parent

loc = Matrix.Translation(pbone.matrix.to_translation())

# Rotation only (strip location and scale)
# TODO: make it work for parent bones with negative scale too
rot = pbone_parent.matrix.to_euler().to_matrix().to_4x4()

s = pbone.matrix.to_scale()        
scale = Matrix()
scale[0][0] = s[0]
scale[1][1] = s[1]
scale[2][2] = s[2]

mat = loc * rot * scale

pbone.matrix = mat

# Force redraw, so transform properties are immediately visible
for area in bpy.context.screen.areas:
    if area.type == 'PROPERTIES':
        area.tag_redraw()


Note that the script has issues with negative scaled parent bones, couldn't figure out how to do it properly.